You'll still get paid for your normal job. It's not like that money would be thrown away, you're not working for free. You go to your job and work because you want
more money.
I think UBI is largely untested and its proponents should proceed with caution. But I also think people are overconfident in assuming that people wouldn't work; the reality is that we don't have a ton of data in either direction.
If the 20th-21st century has taught us anything, it's that consumerism is a very powerful motivator. People like to have money to buy luxuries, most people don't want to sit on subsistence levels of income their entire lives. People want things. They want to buy videogames, and XBoxes, and nice houses, and cars, and iPhones, and to go to fancy restaurants.
Whether consumerism is a powerful enough motivator... I just think people are very over-confident talking about what is definitely going to happen in a system that has basically never been tried before.